Traumatic experiences in early childhood are associated with increased risk for developing mood and anxiety disorders later in life. Low serotonin(1A) receptor (5-HT(1A)R) density during development has been proposed as a trait-like characteristic leading to increased vulnerability of stress-related neuropsychiatric disorders and drug abuse. To assess the relationship between early-life stress and alterations in the serotonin system during development, we used positron emission tomography to measure in vivo 5-HT(1A)R density and apparent dissociation constant (K(D)(app)) in the brains of juvenile Rhesus monkeys exposed to the early-life stress of peer-rearing. In general, 5-HT(1A)R density and K(D)(app) were decreased in peer-reared compared with control mother-reared animals. However, increase in receptor density was found in the dorsomedial prefrontal cortex of peer-reared females. These findings suggest that exposure to an adverse early-life environment during infancy is associated with long-term alterations in the serotonin system and support previous studies suggesting that reduced 5-HT(1A)R density during development might be a factor increasing vulnerability to stress-related neuropsychiatric disorders. Furthermore, alterations in the serotonin system seemed to be gender- and region-specific, providing a biological basis for the higher prevalence of affective disorders in women.